1. Field of the Invention
The present invention relates to an oxide magnetic material.
2. Description of the Related Art
Ferrite is a generic term for any compound including an oxide of a divalent cationic metal and trivalent iron, and ferrite magnets have found a wide variety of applications in numerous types of rotating machines, loudspeakers, and so on. Typical materials for a ferrite magnet include Sr ferrites (SrFe12O19) and Ba ferrites (BaFe12O19) having a hexagonal magnetoplumbite structure. Each of these ferrites is made of iron oxide and a carbonate of strontium (Sr), barium (Ba) or any other suitable element, and can be produced at a relatively low cost by a powder metallurgical process.
A basic composition of a ferrite having the magnetoplumbite structure is normally represented by the chemical formula AO.6Fe2O3, where A is a metal element to be divalent cationic and is selected from the group consisting of Sr, Ba, Pb and other suitable elements.
Recently, it was proposed that La be substituted for a portion of Sr and Co and Zn be substituted for a portion of Fe to increase the coercivity and the remanence of an Sr ferrite (see Patent Document No. 1).
It was also proposed that La and Ca be substituted for a portion of Sr and Co be substituted for a portion of Fe to increase the coercivity of an Sr ferrite while maintaining high remanence thereof (see Patent Document No. 2).
It was also proposed that a rare-earth element such as La be substituted for a portion of Ca in a Ca ferrite and Co be substituted for a portion of Fe to obtain a ferrite magnet with high remanence and high coercivity (see Patent Document No. 3).    Patent Document No. 1: Japanese Patent Publication No. 3163279    Patent Document No. 2: Pamphlet of PCT International Application Publication No. WO2005/027153    Patent Document No. 3: Japanese Patent Publication No. 3181559
As for a Ca ferrite, it is known that a structure such as CaO—Fe2O3 or CaO—2Fe2O3 has good stability and can produce a hexagonal ferrite when La is added thereto. However, the magnetic properties achieved by such a Ca ferrite are comparable to those achieved by a conventional Ba ferrite and are far from being satisfactory. That is why Patent Document No. 3 discloses a technique for increasing the remanence Br and the coercivity HcJ and improving the temperature characteristic of the coercivity HcJ by adding both La and Co to a Ca ferrite (which will be referred to herein as a “CaLaCo ferrite”).
According to Patent Document No. 3, the anisotropic magnetic field HA of such a CaLaCo ferrite, in which a portion of Ca is replaced with a rare-earth element such as La and in which a portion of Fe is replaced with Co, for example, is at most 10% higher than that of an Sr ferrite and can be as high as 20 kOe or more.
According to a specific example of a CaLaCo ferrite disclosed in Patent Document No. 3, if Ca1-x1Lax1(Fe12-x1Cox1)z satisfies x=y=0 through 1 and z=1, good properties are achieved when x=y=0.4 to 0.6. Specifically, Br=4.0 kG (=0.40 T) and HcJ=3.7 kOe (=294 kA/m) when sintering is carried out in the air and Br=4.0 kG (=0.40 T) and HcJ=4.2 kOe (=334 kA/m) when sintering is carried out in oxygen.
If the mole fraction of z is decreased to 0.85 in the compositional formula mentioned above (i.e., x=0.5, y=0.43 and x/y=1.16), Br=4.4 kG (=0.44 T) and HcJ=3.9 kOe (=310 kA/m) when sintering is carried out in the air and Br=4.49 kG (=0.449 T) and HcJ=4.54 kOe (=361 kA/m) when sintering is carried out in 100% oxygen. The latter properties are the best ones for Patent Document No. 3.
Such an Sr ferrite, in which a portion of Sr is replaced with a rare-earth element such as La and a portion of Fe is replaced with Co according to Patent Documents Nos. 1 and 2 (and which will be referred to herein as an “SrLaCo ferrite”) has such good magnetic properties as to be replacing conventional Sr and Ba ferrites more and more often in various applications.
Ferrite magnets are used most often in motors. Therefore, if the magnetic properties of ferrite magnets can be improved, motors can have either increased outputs or reduced sizes. For that reason, there is a growing demand for high-performance ferrite magnets, of which the remanence Br and coercivity HcJ are both sufficiently high. And even the SrLaCo ferrites mentioned above should have their magnetic properties further improved.
A CaLaCo ferrite according to Patent Document No. 3 produces an anisotropic magnetic field HA that is higher than that of an SrLaCo ferrite, has good magnetic properties such as Br and HcJ that are comparable to those of SrLaCo ferrites, and therefore, is expected to expand their applications in the near future. However, the CaLaCo ferrites that have been proposed so far would have not reached their full potential yet and should be further ameliorated to live up to high expectations.